FingaMIDI is a replacement driver for the ADB trackpad found in the Apple G4 Ti PowerBook and probably the Snow iBook, G4 iBook and aluminium PowerBooks. It allows the inbuilt trackpad to be used as a three-axis MIDI controller, sending x, y and z (pressure) data as parameter change messages. OK, its not really pressure data but capacitance, which is proportional to the area of finger(s) or palm in contact with the pad. The trackpad button sends middle-C note on and note off messages.
The software consists of three parts: Replacement trackpad driver kernel extension User-space CoreMIDI driver Preference pane Command line tool to allow switching between modes.
For more information go to the FingaMIDI
Fingamidi is freeware. You can download it here